Azithromycin, 9-Deoxo-9a-aza-9a-methyl-9a-homo-erythromycin A, is a 15 member ring macrolide belonging to a new class of antibiotics termed “Azalides”, due to the incorporation of a nitrogen atom in the macrocyclic ring.
Azithromycin (Formula I) is derived from the 14-membered macrolide antibiotic erythromycin A and shows significant improvement in its activity against gram—Ve organisms compared to erythromycin A (C. J. Dunn and L. B. Barradell Azithromycin: A Review of its Pharmacological properties and use as a 3-day therapy in respiratory tract infections, Drug, 1996, March, 51(3) 483–505).

Azithromycin was first discovered by G. Kobrehel and S. Djokic (U.S. Pat. No. 4,517,359; S. Djokic et al.). U.S. Pat. No. 4,517,359 describes methylation of 11-aza-10-deoxo-10-dihydro erythromycin A (presently called 9-Deoxo-9a-aza-9a-homoerythromycin A) in an excess of formaldehyde and formic acid in a halogenated hydrocarbon, e.g., chloroform or carbon tetrachloride. In the procedure described in U.S. Pat. No. 4,517,359 the isolation of azithromycin comprises extraction of the aqueous layer with a halogenated hydrocarbon solvent followed by evaporation of the solvent. The disadvantages of this process are that (i) a halogenated hydrocarbon is used which is environmentally unsafe and (ii) the isolation of azithromycin involves several cumbersome and/or inefficient extraction and solvent evaporation steps. According to European Patent Application EP 298650, the azithromycin obtained by the process taught in U.S. Pat. No. 4,517,359, is a hygroscopic monohydrate. Because of its hygroscopic nature, this monohydrate is difficult to prepare and maintain in a form having a constant, reproducible water-content, and is particularly difficult to handle during formulation.
The processes taught in U.S. Pat. No. 6,268,489 for the preparation of azithromycin dihydrate, while producing a non-hygroscopic form of azithromycin, have a number of disadvantages:    1. Water immiscibility of the organic solvent mixture (tetrahydrofuran plus hexane) can cause problems in obtaining pure material since crystallization processes are known to afford pure material when the anti-solvent is miscible with the solvent used to dissolve the crude product.    2. The drying process must be very carefully controlled since an increase in temperature will cause the transformation of the non-hygroscopic dihydrate to the hygroscopic monohydrate.    3. The use of low boiling point solvents is complicated by their toxicity and possibility of formation of explosive peroxide during solvent recovery.
Two other synthesis routes, affording azithromycin as a form that should differ from the crystalline ones previously mentioned, have also been described in WO 94/26758 and U.S. Pat. No. 4,517,359. According to such processes azithromycin is obtained by single evaporation to dryness. However, in these prior art documents there is no reference to the crystalline state of the azithromycin thus obtained.
Azihthromycin monohydrate isopropanol clathrate is taught in U.S. Pat. No. 6,245,903. According to this processes the azithromycin monohydrate isopropanol clathrate is obtained from azithromycin, azithromycin monohydrate or azithromycin dihydrate. There are two steps involved in this process, first step is isolating azithromycin in any form (i.e. azithromycin monohydrate or azithromycin dihydrate) from azaerythromycin and second step is the conversion of azithromycin to azithromycin monohydrate isopropanol clathrate.
Being of two steps, the process of U.S. Pat. No. 6,245,903 has the following drawbacks.    1) Substantial total time of process    2) High total utility cost of production    3) Loss of materials during handling.